Counter-Strike is one of the most important and influential PC games in history. It began as a Half-Life mod released in 1999 before Valve bought it out and hired the creators. The version of the game it released in 2000 would over time and tweaks become known as Counter-Strike 1.6, and to this day you will find purists who insist this is the best the game's ever been. Now, pretty remarkably, you can just play it in your browser.
First things first: This is a Russian website that doesn't appear to have any affiliation with Valve. So this is in all likelihood quite illicit, and trying to get the Russian authorities to take it down is going to be a fun time for some poor lawyer in Seattle. On a more practical level you may be worried about malware or trackers. While I'm no cyber-expert, I've run it through three different scanners and it appears clean.
Here then is Play-CS.com, which allows you to play Counter-Strike 1.6 across different regional servers, with all of the game's different modes and various extra functionality. I have to say that our comrades have done a pretty amazing job here: PCG's editor-in-chief Evan Lahti had some bad pings, but for me this runs amazingly well. I've played several rounds and modes across different servers, the functionality seems great, it feels exactly like CS 1.6, and I just got headshot by Bob Dylan (clearly taking a break from his never-ending tour).
It does seem like a bit of a passion project, and if you loved CS 1.6 back in the day this is sure to provide half-an-hour's fun nostalgia. It's clearly a bit dodgy in terms of not having anything to do with Valve, but the fact someone's made a game from 1999, one of the most iconic games in PC history, playable in a browser with full online functionality and no issues, is seriously impressive.
The 'real' Counter-Strike is still going strong in the form of Counter-Strike Global Offensive, which recently celebrated its tenth anniversary (and was made to run at 4,000 FPS by one of its greatest devotees). And while it's not quite on Doom's level, it does turn up in even odder places than browsers: Like this Nintendo DS port, which absolutely slaps.